Brake problems - pedal dropped

When I was out driving yesterday, the brake pedal suddenly sank to the floor. When I pressed it again it was fine. Then after a while it went down again, only to stiffen up at the second try.

I haven't had the time to look into it, but does anyone have an idea what it is? I'll probably replace the brake fluid and bleed the system just for the heck of it, but I hope I won't need to repair calipers or anything. Btw, the braking power is OK when the pedal is behaving.

[ctaylor738]

Unless you have an obvious leak or loss of fluid from a caliper, this sounds like a master cylinder that's on it's way out. If you are going to flush and bleed it isn't very much extra work to replace the MC.

[300SDog]

Yep, the MC is my guess too. I've seen them wear out on 3 of my MB's over the past 25 yrs, usually at/near 130,000 miles - dunno how many kilometers.

Fresh DOT 4 fluid flush will make the Master Cylinder slip even worse.

I got lucky and bought a brand new MC for my 240D on ebay at $68 (US) couple of months ago. New MC includes grommets for the reservoir and rubber seal for the power brake unit.

Toughest part of the job was bleeding the system. Old fluid was practically black. I ended up buying "Speed Bleeder" valves for the calipres: http://www.speedbleeder.com/

Another neat trick is cleaning out the clear plastic brake fluid reservoir. First I used paint thinner (mineral spirits) then regular dishsoap and water, making sure it was completely dry from sitting in the sunshine before installing it.

OK, I've been driving the car for some time now (who says Swedes are obsessed with safety) and the brakes work without a problem. I can easily lock all wheels when I brake hard. But maybe I should just replace it anyway. I discovered that a MC on www.*************** could be mine for the neat sum of $75 with a shipping cost of $25 to Sweden. That's less than half of what a similar ATE MC would cost at a Swedish supplier, and who knows what MBZ wants. Thank God for strong currencies

[300SDog]

Yep, that's completely understandable to me. The MC that I replaced last Summer was on it's way out for almost 2 yrs.

First the pedal could sometimes slowly be forced to the floor under pressure while standing still. Then more than a year later double pumping the pedal became standard procedure while braking gently, but under sudden stops slamming the pedal once with full force always worked as the discs could be locked up making the car skid to a halt.

Finally, I flushed the fluid nearly 2 yrs after symptoms began. Big Mistake!!! That's when all hell broke loose and the pedal started going straight to the floor no matter what kind of pressure was applied. Even double pumping the brakes didnt do alot to improve the situation. It got to the point where I was stepping on the emergency brake to prevent bumping into people at red lights or rolling into other cars while parking. Less than 2 weeks after flushing the system, I was forced to install a brand new Master Cylinder!

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Last edited by 300SDog; 10-12-2004 at 12:42 PM.

Thanks DieselDog, always there to solve my problems!

I think I'll skip the master cylinder I saw on German eBay for about $20... The harsh Scandinavian winter comes to my rescue!

[Tomguy]

Originally Posted by 240DieselDog

For sure I believe that fancy brakes such as the ridiculous ABS system are not the kinda thing that a skillful driver requires at all.

True, I would agree with that 100% if not for one thing:
In a panic stop, half the people out there would just SLAM the pedal to the floor and their front wheels would lock up, losing their ability to steer their car. I wouldn't want to be in the other lane of traffic with one of these people headed towards me